1. Field of the Invention
The present invention relates to an ultra-compact type motor with the high power output.
2. Related Background Art
A motor formed as a compact type motor is conventionally well known as, for example, a compact cylindrical step motor, as shown in FIG. 16. A stator 102 of the step motor is arranged in a manner that a stator coil 105 is coaxially wound on a bobbin 101 fixed and sandwiched in an axial direction between two stator yokes 106, and stator gear teeth 106a and 106b are alternately disposed on the stator yoke 106, on the inner peripheral surface of the bobbin 101 in the circumferential direction, and that the stator yoke 106, integrated to the stator teeth 106a or 106b, is fixed to the case 103.
A flange 115 and a bearing 108 are fixed to one of two sets of cases 103 and another bearing 108 is fixed to the outer part of the cases 103. A rotor 109 includes a rotor magnet 111 fixed to a rotor shaft 110 and the rotor magnet 111 defines a radial gap portion together with the stator yoke 106a of the stator 102. Further, the rotor shaft 110 is rotatably supported between two bearings 108.
However, the above-mentioned conventional compact step motor has the disadvantage of having large outer dimensions since the case 103, the bobbin 101, the stator coil 105, the stator yoke 106 and the like are coaxially disposed around the periphery of the rotor 109. Further, the magnetic flux generated by energization of the stator 105 mainly passes through an end face 106al of the stator teeth 106a and an end face 106bl of the stator teeth 106b as shown in FIG. 17, so that it does not effectively act on the rotor magnet 111, whereby the power output of the motor does not become higher.
The present inventors proposed a motor in which such problems had been overcome, as U.S. Pat. No. 5,831,356. The proposed motor has a cylindrical rotor comprised of a permanent magnet divided into equal parts in a circumferential direction and alternately polarized to different magnetic poles. A first coil, a rotor, and a second coil are disposed in an axial direction of the rotor in this order and a first outer magnetic pole and a second magnetic pole, which are excited by the first coil, are opposed to the outer and inner peripheral surfaces of the rotor, and a second outer magnetic pole and a second magnetic pole, which are excited by the second coil, are opposed to the outer and inner peripheral surfaces of the rotor. A rotary shaft that is a rotor shaft may be removed from within the cylindrical permanent magnet.
Thus configured motors may have a high power output and small outer dimensions. However, machining of the magnetic pole teeth is difficult because of the small diameter of an outer magnetic pole. Further, a small-diametered motor free from vibration and having a stable power output are required.
Thus, the present inventors recently proposed a motor in which the inner magnetic pole has a shape with improved machinability in U.S. patent application Ser. No. 08/994,994. Further, the present inventors proposed a rotor in which output transmission means, such as a gear and pulley, is easily mounted on a rotary shaft with a small diameter, in U.S. patent application Ser. No. 09/022,474. Furthermore, it has been required to obtain a stable and high power motor that is configured as an ultra-compact type motor.